Austenitic steel reformer tubes are used in many chemical processes. Examples include tubes used to produce ammonia, methanol, hydrogen, nitric and sulfuric acids, and cracking of petroleum. Reformer tubes, also called catalyst tubes, are one of the highest cost components of such plants both in capital and maintenance. A typical installation consists of several hundred vertical tubes. These tubes represent a significant cost for replacement and can be a major source of plant unavailability if unplanned failures occur.
Such tubes are typically subjected to high temperatures, temperature gradients, pressure changes and contact with corrosive substances. Under such situations creep, metal dusting, and surface irregularities frequently develop. Creep is a diffusion related process that develops gradually. The signs are not noticeable by reformer operator. Creep forms microscopic voids which coalescence and eventually form creep fissure (cracks). If left untreated, creep will develop into cracks that will propagate leading to catastrophic failure of the tube during service.
The plant operator is faced with balancing production needs against tube life and risk of tube failure. During plant operation the catalyst filled tubes are externally heated to allow the reforming reaction to occur. One of the major concerns in plant operation is that the reformer tubes operate at a highly elevated temperature (up to 1150-1200° C.) such that they are susceptible to the failure mechanism referred to above as “creep”. This condition exists due to the elevated temperatures and stresses imposed by internal pressure, thermal gradients, and mechanical loading cycles. Being able to identify and locate such damage in its early stages is essential for optimizing plant operation and extending the tube's useful service life.
Known Non-Destructive Testing (NDT) methods based on intermodulation measurements are used to find nonlinear conductive materials contained in a non conductive substrate. A different method is needed to deal with non-linear magnetic materials contained in a conductive substrate. Existing NDT methods for austenitic steel are based on laser shape measurement, eddy current testing for surface cracks, and ultrasound testing for subsurface cracks. These methods are useful, but tell little or nothing about changes early in the life of the material. In addition, the existing methods require knowledge of the initial conditions of the material and are subject to error due to changes in surface conditions.
Conventional NDT inspection techniques currently applied to reformer tubes are geared to finding creep damage in the form of internal cracking. However, with the trend towards larger tube diameters and longer intervals between turnarounds, the detection of such defects may not allow for sufficient time for forward planning of tube replacements. Also, such “end of life” techniques do not allow any differentiation between the “good” tubes and the “bad” tubes. Early detection of underutilized tube life can prevent the lost opportunity on both unrealized production through running them too cool and tube life “giveaway” if good tubes are discarded prematurely.
Typically, destructive testing is used on a small number of tubes removed from the reformer to try and determine the absolute life remaining. Whatever method is used, the results are used on a sample size that is not statistically valid. It is preferable that all the tubes be surveyed with a NDT technique to characterize their relative condition.
Reformer tubes undergo creep strain, in the form of longitudinal and/or diametrical growth, from the first day that they are fired. Measuring the creep elongation of such tubes is the most popular deterioration detection method in routine use today, but this method is very inaccurate for monitoring in service tube deterioration. This because there is no known method for measuring the local longitudinal growth, just total growth which is averaged over the whole length of the tube.
Measuring the diametrical growth is more accurate but could can lead to inaccurate measurements early in the service life of a tube due to the scale effect. That is, accurate measurement of circumferential growth is complicated by the growth and sloughing of a corrosion layer (scale) on the surface of the tube which mimics diametrical expansion. Measuring the diametrical growth also requires tube climbing equipment.
The ability to accurately measure and record tube deterioration means that the tubes' condition can be monitored on day one. Therefore, not only can individual tubes be retired from service at an appropriate time, but also the reformer as a whole can be assessed for performance.
To get an idea of the scope of the problem to be solved, one should note that, at present, ArcelorMittal has 8 reformers that use about 2,500 reformer tubes. Tubes are quite expensive, costing more than $30,000 each, plus catalyst costs which doubles the tube cost along with cost of installation. Reformers operate continuously from 2 to 5 years between cold shutdowns.